Multi laterals (multiple smaller boreholes extending from a main borehole) have been drilled in a number of locations in recent years. The main motivation for this is to improve the contact with the reservoir, while minimizing the total drilling cost. Their use can also be motivated by limited template availability in off-shore platforms. In most cases, drilling of multi laterals requires complex operations with multiple trips consuming a lot of rig time. The completion of the lateral is also a complex operation: the junction to the parent well is a key element that defines the quality of the lateral and the method to control the production.
Several techniques have been developed recently to drill small laterals faster with less support from a drilling rig. Several methods have been proposed to drill laterals from a system operated via a wireline cable. Based on such methods, the lateral can be drilled without a drill string or coil-tubing as a link to surface. Examples of such techniques can be found in EP 1 559 864, WO2004072437 and WO2004011766. Multiple drilling tools are also known for drilling extended perforations (typically 1 meter long by a few centimeter diameter).
One example of a prior art system is the SCORE100 tool from Corpro Systems Ltd which operates from a main bottom hole assembly (BHA). The main BHA includes a modified drill collar which contains an integrated whipstock. When the special drill collar is positioned at the required depth in the main hole, a special small diameter BHA can be lowered inside the main drill string on a wireline cable. This small BHA contains a core barrel, a small drilling motor to rotate the core barrel, an anchor, a pushing system to generate forwards movement (penetration (ROP) and weight-on-bit (WOB)), and inflatable packer to divert the flow in to the small motor. Such a system can then anchor itself inside the collar, and push itself forward with a jacking system which generates WOB. Surface pumps generate mud flow which activates the small motor and cleans the small lateral. At the beginning of the axial displacement, the tip of the small BHA is pushed outside the main BHA by the integrated whipstock and then enters the open-hole and then the formation. The axis of the lateral is typically 3 to 6 degree inclined versus the axis of the main well. Such a system can drill a small hole up to 100 feet. Typically such a system is used for coring. The wireline cable and tool provides down-hole control of the processes especially for ROP and WOB. It also controls the anchoring of the system in the collar to ensure the crawling movement. When the lateral or the coring process is completed, the small BHA is fished with the wireline cable. The collar window is typically plugged with an aluminum ball which is drilled out by the small BHA. EP 1 247 936 describes other details of this technique.
It is an object of the invention to provide a technique which can be used for effective drilling of laterals which does not require such significant interruptions to deploy the small BHA.